Have, any of you attempted it, and with any sucess? I want to redo my boats upholstery and after doing some research online it doesn't seem like it would be that hard, maybe a little practice with a sewing maching. Does anyone have any advice?

Which type/parts/pieces of upholstery are you considering re-doing?

What kind of boat?

In some cases, it's cheaper to just buy new vs re-upholstering - depending on what it is.

it's a 19ft open bow maxum, I planned on redoing all of the seats in it,

Take it apart a piece at a time (notice I said take it apart don't tear it apart), use as patterns and go back together with it. I've done several over the years. It does help if you can sew a straight line, if not talk wife or friend into helping. If all else fails practice sewing straight lines before you start.

An open bow negates the idea just buying new. There's too many "custom" pieces.

I had a buddy that owned an upholstery shop and I used hang out there and watch him work. It's not rocket science. I think the biggest issue for someone doing it on their own would be having a sewing machine that's capable of the job.

You can do it yourself, but to make it look acceptable is a different thing altogether. I had an old open-bow Tahiti jet boat that I attempted this on years ago, and it turned out like crap. I wound up taking the pieces to a small reputable shop, and it was well worth the labor cost, especially at resale time.
The best work I did was in the re-manufacturing of the seat frames themselves, which had all but completely rotted away. I made new ones out of marine plywood and coated them with multiple coats of Man-O-War. I would guess that those frames are still good today.
One very strong recommendation if you do decide to give it a go, is to use monel or stainless staples. I tell the same to those folks who re-carpet their trailer bunks.

+1 on the stainless staples.

That and don't get stingy with the contact cement. Staples and contact cement works mo betta.

If you're using the existing upholstery as a pattern make sure to cut your new pieces a little smaller to allow for stretch over the life of the seats. Otherwise they will get loose and sloppy looking after a pretty short period.